1. Field of the Invention
The present invention relates to an AM radio receiver, and particularly to an AM radio receiver with no tracking error.
2. Description of the Prior Art
AM radio receivers which receive an AM broadcast wave (RF signal) transmitted from a broadcast station are known as described on page 73 in the 1988 "Sanyo Semiconductor Data Book, bipolar integrated circuits for car audio".
An example of a prior art AM radio receiver is shown in FIG. 2 , which comprises: an antenna damping circuit 2 which attenuates the RF signal received by the antenna 1; an RF amplifier 5 including an FET 3 which amplifies the RF signal and an AGC transistor 4 having the base thereof applied with an AGC signal; an RF tuning circuit 6; a mixing circuit 8 which mixes an RF signal obtained from the RF tuning circuit 6 and a local oscillation signal obtained from the local oscillation circuit 7; an IF (intermediate frequency) amplifier 9 which amplifies the IF signal obtained from the mixing circuit 8; and an AM detector 10 which detects the output signal of the IF amplifier 9 by the amplitude demodulation.
However, according to the prior art AM radio receiver as thus described, the resonance frequency of the RF tuning circuit 6 and the resonance frequency of the local oscillation circuit 7 must be designed to have a difference equal to the frequency (450 kHz) of the IF signal. Usually, in the prior art AM radio receiver, of all the received frequency band (e.g., from 522 kHz to 1602 kHz), the frequency adjustment is made only at the tracking points (e.g., 600 kHz and 1400 kHz). Thus, at other frequencies the frequency difference is not precisely equal to 450 kHz.
Thus, in the prior art AM radio receiver, there have been observed such a disadvantage that a drop in gain of the RF amplifier is induced. Furthermore, in a stereo AM radio receiver, such a drop in gain is accompanied by a deterioration in separation. Also, during automatic tuning, variations in the stopping sensitivity may also occur. These problems are particularly pronounced at frequencies (522 kHz 600 kHz) lower than the first tracking point (600 kHz) and at frequencies (1400 kHz-1602 kHz) higher than the second tracking point (1400 kHz).
The untuned AM radio receiver as shown in FIG. 3 has been proposed as one means for solving these problems. In FIG. 3 the RF signal amplified by the FET 3 is applied to a first mixing circuit 8 in an untuned state from the collector of the AGC transistor 4. In the first mixing circuit 8, the RF signal and the output signal of the local oscillation circuit 7 are mixed, and a first IF signal of 10.7 MHz is generated at the output terminal of the first mixing circuit 8. Then the first IF signal is mixed with the output signal of the second local oscillation circuit 12 by the second mixing circuit 11, and a second IF signal of 450 kHz is generated at the output terminal of the second mixing circuit 11. The AM radio receiver as shown in FIG. 3 increases selectivity with this double conversion, and is able to eliminate tracking error because it has no RF tuning circuit. Furthermore, image interference is extremely low because the first IF signal frequency is set at 10.7 MHz.